The brainstem is a small yet very important component of the brain that serves a multitude of functions affecting the whole body. The structure is located on the posterior side of the brain and connects directly to the spinal cord. The brainstem is essential for regulating basic processes including breathing, heart rate, sleep, and digestion. Furthermore, it transmits information between the central and peripheral nervous systems through nerve tracts. The brainstem supplies many cranial nerves to the face and neck, allowing for sensory and motor conduction. Conversely, it also receives sensory and motor input from peripheral nerves for appropriate interpretation by the brain. Through its vast range of functions that are crucial for living, the brainstem proves to be a vital part of the central nervous system.
Anatomy and Physiology
The brainstem is positioned at the base of the cerebrum, above the spinal column, and anterior to the cerebellum. It consists of three distinct structures called the midbrain, pons, and medulla oblongata.
The midbrain, also called the mesencephalon, is the superior-most portion of the brainstem. Its name is derived from the Greek terms mesos, meaning middle, and enkephalos, meaning brain. The midbrain is located adjacent to the cerebral hemispheres, just below the cerebral cortex and above the pons. This structure is further organized into parts known as the corpora quadragemina, cerebral aqueduct, tegmentum, and cerebral peduncles. Within these components exist many nuclei (collection of cell bodies) and fasciculi (clusters and bundles of axons). One well-established nucleus of the midbrain is the substantia nigra, which releases the neurotransmitter dopamine that plays an important role in movement control. Other functions of the midbrain include regulation of arousal, sleep/wake cycles, and temperature. This structure also contains auditory and visual reflex centers to control proper hearing and vision.
The pons is an approximately 2.5-centimeter-long section of the brainstem, located between the midbrain and medulla oblongata. Based on its location it was given the name pons, which is derived from the Latin term meaning bridge. Functions of the pons include regulation of the sleep cycle and the development of dreams. Also, the pons contains four cranial nerves (V, VI, VII, and VIII), which it supplies to the face and neck. Through this innervation, the structure is essential for sensory functions such as taste, facial sensation, and hearing, as well as motor functions such as eye movement, facial expression, chewing, and swallowing. The pons also plays a role in bladder control, posture, and balance. Located within the pons is the pneumotaxic center, a nucleus that is responsible for regulating the switch from inspiration (breathing in) to expiration (breathing out). Therefore, the pons is also essential in proper respiratory function.
The medulla oblongata is the inferior-most part of the brainstem, located adjacent to the spinal column. In this position, it acts as a connector of motor tracts between the higher centers of the brain and the spinal cord. The superior sections of the medulla oblongata also form a wall of the fourth ventricle, a compartment where cerebrospinal fluid is maintained. Key functions of the medulla oblongata include regulation of basic yet fundamental autonomic processes. These include respiration, vasodilation (blood vessel relaxation), and cardiac function. Additionally, the medulla oblongata controls reflexes such as vomiting, sneezing, coughing, and swallowing.
Due to its vast range of functions, damage to the brainstem can manifest in a variety of serious outcomes. Diseases that damage the brainstem can do so through bleeding, tumors, formation of plaques, lack of oxygen supply, or demyelination (unsheathing of neurons’ axons). If a cranial nerve within the brainstem is affected, the subsequent sensory and motor stimuli will not be experienced or performed properly by the face and neck. Additionally, if a nerve tract is altered, there may be improper perception of peripheral stimuli. Based on the presentation of symptoms, whether they are visual, auditory, speech, or otherwise, the location of damage can be determined. Regardless, damage to the brainstem may be irreversible and requires immediate, invasive medical attention. Therefore, preservation of a healthy brainstem is vital, as this structure is responsible for many basic functions of life.
See also: Auditory System; Autonomic Nervous System; Inferior Colliculus; Superior Colliculus; Taste System
Haines, Duane E., & M. D. Ard. (2013). Fundamental neuroscience for basic and clinical applications. Philadelphia, PA: Elsevier/Saunders.
Tortora, Gerard J., & Bryan H. Derrickson. (2012). Principles of anatomy & physiology. Hoboken, NJ: Wiley.
Urban, Peter P., & Louis R. Caplan. (2011). Brainstem disorders. Berlin: Springer.